Hearing aids in general comprise one or more microphones for converting sound pressure to an electrical input signal. By placing two microphones spaced apart on each hearing aid the input signals from these two microphones may be used to perform a directionality focus of the hearing aid. Generating a directionality focus of a hearing aid improves the user's ability to hear sounds originating in front of the user, which is particularly advantageous in noisy surroundings.
European patent application EP 1 458 216, which hereby is incorporated in below specification by reference, discloses a system and method for matching hearing aid microphones. The system comprises a first microphone connecting to an infinite impulse response (IIR) filter controlled according to the following transfer function:
                    M                  i          ⁢                                          ⁢          n                            M        out              =                                                      p              1                        ⁡                          (                              X                p                            )                                ·          z                +                              p            0                    ⁡                      (                          X              p                        )                                      z        +                              q            0                    ⁡                      (                          X              p                        )                                ,where Min is the microphone input, Mout is the output of the IIR filter, z is the frequency variable, p1, p0 and q0 are functions of controlling parameter Xp.
The functions p1, p0 and q0 are described in European patent application EP 0 982 971 as abbreviations of a microphone model. The functions describe poles and zeros of the characteristics of a microphone response to frequency variances.
The controlling parameter Xp ensures that the difference between acoustic response of the first microphone matches acoustic response of a second microphone. Xp is calculated by comparing a band-pass filtered and amplitude compensated output of the IIR-filter with a band-pass filtered output of a reference microphone. The system utilises level measuring means for establishing a level of the first microphone's signal and a level of the reference microphone's signal. These levels are feed to a subtraction unit subtracting the levels. This result is forwarded to a threshold unit, which enables the generation of Xp in an Xp-generator when the result is above a certain threshold.
The system only presents a single loop simultaneously adjusting polls and zeros, which causes the matching of the microphones to be inadequate. Hence there is still a need for further improvements in achieving matching of microphones in a hearing aid.
Further the amplitude compensation is executed independently on the output of the IIR-filter without direct reference to the reference microphone. Hence this microphone matching requires still further improvements.